1. Field of the Invention
This invention relates to bending methods for forming sheet metal parts and more specifically for calculating springback when forming parts by hydroforming with a fixed die and hydraulic pressure exerted by a flexible diaphragm.
2. Description of Prior Art
With the advent of metal airplanes, it became necessary to find a method of forming complex sheet metal parts economically. Traditional stamping methods, also referred to as draw forming, involve high tool costs that could not be justified for the relatively small quantities of parts produced. Traditional stamping required two mating precisely aligned dies. Hydroforming, also referred to as fluid forming, involves only a single die wherein the hydraulic pressure was applied against a flexible diaphragm, which forms the sheet material against a die.
Various other metal forming methods are utilized in the various industries such as a stretch press, wherein the work piece is stretched over a single die. The brake press is another commonly used method, which deals with more simplified two-dimensional bends.
When a metal workpiece is deformed during a metal forming operation, the deformation thus given has two components—elastic and plastic. Upon removal of the forming load, while the plastic component remains unchanged, the elastic component is recovered. The magnitude of this recovery is called springback. All metal forming methods thus have to deal with the springback problem. During sheet bending, this recovery or springback is manifested in the workpiece in the form of an increased part angle and radius of bend than that desired. The shape of the workpiece springs back to a shape, which is almost never the shape optimally desired nor the shape of the bending die. In the prior art this required re-cutting the forming die numerous times, which is very costly and time consuming.
One method to solve this problem has been to cause the workpiece to be excessively bent in the bending direction such that upon springback the workpiece can assume the proper shape. This method requires the die designer to guess at the shape of the bending die, which can be very costly if incorrect.
Solving the springback problem has been addressed in various other metal forming methods such as the patent to Ewert, U.S. Pat. No. 4,989,439 in a stretch press process; the patent to Jones, U.S. Pat. No. 4,802,357 and Ooenoki et al, U.S. Pat. No. 6,161,408, both of which deal with methods for compensating for springback in the brake press field of metal forming. The patent to Yamano et al, Pub. No. US 2003/0061852 deals with calculated springback in the conventional draw forming method of metal forming.
Research to predict springback in hydroforming, has been very limited and is exemplified in a publication entitled “Sheet Metal Forming in the Quintus® Fluid Cell Press” published in April of 1980 and authored by Eric Enroth, published by the Quintus Press Department. This publication provides some rules of thumb for springback allowances for different materials irrespective of geometric or process conditions. Commonly used springback prediction tools in the industry are based on experiments done for a specific bend angle (90 degrees) for specific materials, and springback charts obtained by doing tests on specific geometric conditions. The springback is predicted for different geometric conditions on the basis of limited data and arbitrary extrapolations; therefore, their prediction is not precise. Process parameters are not considered in these methods and any process variation during manufacturing alters springback. There is no single tool available that can accurately predict the total springback for all parts irrespective of material, process and geometric condition.